This invention relates to heat recoverable articles and more particularly to heat shrinkable articles that can be positioned around a cable, pipe, or connector at a joint or splice and then be caused to heat recover in place to thereby encapsulate said splice.
There are many applications where it is desirable to provide a sealing, insulating or protective encapsulating or enclosing member for elongated objects such as cables, pipes or the like. Such encapsulation is particularly important where pipes, cables and the like are joined or spliced, particularly when a joint involving a plurality of pipes, cables, etc. is involved. In many instances, the ends of elongated objects (hereinafter the term cables will be used, although the invention is, of course, useful for enclosing or encapsulating pipes, cables, ducts, conduits and the like elongated substrate members) are not conveniently accessible to a sealing member having a tubular configuration, and to place such a tube thereover, the elongated object must be disconnected or displaced from its original position. To overcome this shortcoming of heat recoverable tubes, closure members suitable for wrapping around the elongated objects have been developed. See for example, U.S. Pat. Nos. 3,379,218 to Conde and 3,455,336 to Ellis or U.S. Pat. No. 3,770,556 to Evans, all assigned to the same assignee as the present application. These so-called "wrap-around" closures can be installed around an elongated member without access to a free end thereof. There is nevertheless a significant need for a closure, hereinafter referred to as a "splice case," suitable for enclosing electrical cable joints, splices, and the like, which provides effective environmental protection, in particular, for splices involving more than two incoming cable ends and/or splices between different sizes of cables but which can be applied without access to a free end of the cable.
This invention is directed to such a heat-recoverable, splice case which can, in optional embodiments, accommodate a plurality of cables of differing size, i.e., outside diameter, which can be removed and reapplied to a splice and which does not require access to a free end of the cable. The present design is not referred to as a "wrap-around" since it encapsulates a splice in a somewhat different fashion from the aforementioned "wrap-around" closures. In alternative embodiments the splice case of the present invention utilizes either a "clam shell" or separate base plate and cover member design.
In one embodiment the present invention contemplates a splice case which will recover and encapsulate a cable or other splice when subjected to an external heat source such as a propane torch or hot air blower.
For reasons hereinafter set forth, in a preferred embodiment the splice case of the present invention includes integral heating means. That is, the splice case contains an integral electrical resistance heating element which, when connected to an external electric current source, generates sufficient heat to cause the splice case to recover and encapsulate the splice. Many of the applications for such splice cases contemplate in-field use, where the location of the splice is difficultly accessible or in a potentially hazardous environment, and great care must be taken in installing the splice case.
In the connection of overhead telephone cables, or in underground mines which may contain flammable gases, the use of an open flame torch is often not only dangerous, but sometimes strictly prohibited. Under such circumstances a wrap-around closure, i.e., splice case, which does not require the application of external heat, particularly a flame, would be particularly advantageous. Considerable effort has heretofore been expended to devise a closure means which is not only economical to fabricate but quick, easy, and safe to install. To this end it would be desirable to have a heat recoverable splice case which does not require an outside heating source, but instead may be caused to recover by connection to an electric power source, such as a 12 or 24 volt battery, or a 115 volt A.C. outlet, and which when connected to such a power source, will cause heat recovery of the splice case and also activation of an adhesive or sealant on the inner surface thereof.
In formulating conductive heating compositions for use in the splice cases of this invention, uniform heating of the composition is important. In addition, for applications where the heating element must cause heat activation of an adhesive or sealant, as well as heat recovery of the article, relatively high temperatures on the order of 120.degree. C to 200.degree. C must be not only obtained, but also carefully controlled. If higher temperatures beyond that necessary for heat recovery of the splice case and adhesive activation occur, then permanent damage to the sealing article, i.e., the splice case, and/or to the part to be sealed, e.g., the substrate cable, may result, such damage frequently not being apparent by visual inspection of the recovered splice case and immediately adjoining areas of the cable.
Of course, thermostats and/or other heat control devices may be employed to control the temperature of the recovered article. But for many applications, this defeats the purpose of using a self-contained. i.e., self-heating, closure system, in that expensive, sensitive and/or bulky external temperature control devices must be employed in what are sometimes virtually inaccessible places. Moreover, the temperature sensed by the thermostat is only that of its immediate environment. Other areas of the case may be at considerably lower or higher temperatures.
In recent years a new approach for electrical heating appliances has seen the use of self-regulating heating systems which utilize plastic materials exhibiting PTC (positive temperature coefficient) of resistance characteristics. Such materials comprise crystalline thermoplastics plus a conductive particulate filler.
The distinguishing characteristic of these PTC materials is that upon reaching a certain temperature, a rapid rise in resistance occurs. The temperature at which the resistance increases sharply is often designated the switching temperature (T.sub.s) since the current at that point tends to switch off, thereby preventing permanent damage through further temperature increase to the heating article itself or any article being heated thereby.
Although a number of theories have been propounded for the sharp rise in resistance of the PTC material at about its crystalline melting point, it is generally believed that such behavior is related to the difference in thermal expansion of the conductive filler and the thermoplastic matrix materials at the melting point. For a more detailed discussion of a number of alternative mechanisms to explain the PTC phenomena, see "Glass Transition Temperatures as a Guide to the Selection of Polymers Suitable for PTC Materials," J. Meyers, Polymer Engineering in Science, November, 1973, vol. 13, no. 6.
Most self-regulating heating apparatus utilizing a PTC material contemplate steep R = f(T) curves at about the T.sub.s temperature so that above this certain temperature the device will in effect completely shut off, while below this temperature, relatively constant wattage output at a given voltage is achieved. At low temperatures, the resistance is at a relatively low and constant level and thus the current value is relatively high for any constant voltage (E = IR). This power is dissipated as Joule heat (I.sup.2 R), thereby warming up the material. The resistance stays at this relatively low level until the T.sub.s temperature, where a rapid increase in resistance occurs. With the increase in resistance, there is a decrease in power, thereby limiting the amount of heat (I.sup.2 R) generated and for extremely steep R = f (T) curves, heating is in effect stopped. Then, upon a lowering of the temperature, the resistance drops, thereby increasing the power output.
Thus, when an applied voltage is directed across a PTC heating element, the Joule heat, (I.sup.2 R) causes rapid heating of the PTC element up to its switching temperatures, after which little additional temperature rise will occur due to the steep increase in resistance. On account of the steep resistance rise, the heating element will theoretically reach a steady state at about the switching temperature, thereby self-regulating the heat output without resort to fuses or thermostats.
Thermoplastic PTC materials contemplated by the prior art are highly crystalline and exhibit a T.sub.s at about the crystalline melting point, however, most such materials in fact show a "curl over" effect, i.e., the resistance drops again at temperatures much above the melting point. This decrease in resistance above the melting point is generally undesirable, especially in cases where the PTC material is itself heat recoverable, or is used in intimate proximity with a heat recoverable material to effect recovery thereof, since under such circumstances it is preferred to heat the heat shrinkable material as rapidly as possible up to its melting point (i.e., by means of high power densities) and thereafter keep the heater temperature very slightly above the melting point of the thermoplastic constituent(s) of the heater to facilitate rapid and effective shrinkage of the heat recoverable article.
However, heat recoverable articles, such as are comprehended by the instant invention, are intended in use to encapsulate and environmentally seal splices between, for example, telephone cables, by shrinking down onto and bonding securely to the cable jacket which generally comprises a low melting, partly crystalline, thermoplastic composition such as a carbon black loaded ethylene-vinyl acetate polymer. Such cable jackets are almost always uncrosslinked and, therefore, will flow and distort readily if the heater causes said jackets to heat to too high a temperature (i.e., over their melting points) during the time at such temperature needed to activate the adhesive. Even more serious results can occur with a heater which does not very positively "shut off" if, as may happen, the artisan making the splice forgets, or otherwise omits, to disconnect the battery (or other electrical power source) from the heat shrinkable article. Under such circumstances, it is conceivable that the PTC heater could remain energized for periods of 1 hour, or even more, for example, although the encapsulation process may take only, for example, 10 minutes. The above considerations are made even more critical if, as often happens, the individual conductors within telephone cables are each insulated with similar thermoplastic compositions. Any distortion of such conductor jackets is unacceptable, as it causes that section of the cable to become nonfunctional. Thus, there exists a need for a heater for articles such as those disclosed and claimed in the instant application which undergo a steep and extensive increase in resistance above the T.sub.s of the article heater element and whose resistance continues to rise as the temperature of the heater is increased above the melting point of the thermoplastic constituent, rather then "curling over" i.e., declining more or less steeply as occurs with most, if not all, prior art heaters. It should be noted that the "curl over" phenomenon was not generally recognized by most prior art workers.
Furthermore, it has heretofore been generally believed that conductive polymeric materials exhibiting PTC characteristics did not have sufficient heating capacity to cause recovery of relatively thick sections of heat recoverable materials as contemplated for the splice case of this invention, nor the capacity to activate the high temperature adhesives also contemplated by this invention.
The shortcomings of the prior art PTC material for articles such as the splice case of the current invention can be overcome by the use of the compositions disclosed in co-pending, commonly assigned, application Ser. No. 510,035, filed Sept. 27, 1974 (now abandoned), and application entitled "Positive Temperature Coefficient of Resistance Compositions" filed on even date herewith having Ser. No. 601,639 and by utilizing constructions of the type set forth in application Ser. No. 510,036 filed Sept. 27, 1974 (now abandoned) and in application Ser. No. 601,638 concurrently filed herewith entitled "Layered Self-Regulating Heating Article." However, it should be noted that although prior art PTC materials are preferred, they are suitable for use in the splice case of the present invention under many circumstances.
During the operation of telephone cables, especially when the individual conductors are wrapped with a paper-based dielectric, it is required that moisture be excluded since, if the moisture content of the wire insulation increases beyond a certain relatively low, critical level, the electrical characteristics of the wire are unacceptably impaired. For this reason it is customary when prior art telephone cable splice cases are installed for the artisan to place in the case just prior to assembly, a small paper bag of desiccant (usually silica gel) in an amount sufficient to maintain the interior humidity of the case at a very low level over the lifetime of the splice case, whatever the outside humidity. In a typical instance, about 2 oz (.apprxeq.50 g) silica gel might be used. As might be expected, this technique is appallingly craft sensitive, since besides outright ommission to place the bag in the completed splice case structure, the bags (which are customarily sealed for shelf storage while awaiting use) are sometimes left in an unsealed condition for extended periods of time before emplacement, or, in the extreme, even dropped into water or wet mud and emplaced nonetheless. A preferred embodiment of this invention alleviates this problem, as will be explained hereinafter.
It is an object of this invention to provide a heat recoverable closure system which effectively encapsulates a plurality of cables of varying size.
It is a further object of this invention to provide a heat recoverable closure assembly which may be inserted over or wrapped around cables and which has sufficient self-heating capacity to seal such cables without resort to outside heating sources.
It is another object of this invention to provide a reliable self-heating closure system which will self-regulate and not overheat thereby causing permanent damage to the article encapsulated, nor on the other hand shut off at a temperature less than that which it was designed to reach.
Further objects and advantages of the present invention will be apparent from the more detailed discussion and description thereof which follows.